The first radars for weather detection employed a single, fixed polarization transmission signal and receivers that were optimized to receive that particular polarization.
These first radars usually employed substantially linear polarization. Horizontal or Vertical polarization was most commonly used. It was less common to find single polarization scanning radars for weather detection that employ circular polarization or linear polarization at other than Horizontal or Vertical for a number reasons. One reason is that water droplets are asymmetric but aligned with vertical with respect to the surface of the Earth.
Since the first radars, it has been discovered that employing two or more fixed, orthogonal polarization signals in a radar is helpful in classifying and distinguishing targets such as distinguishing between essentially spherically symmetric ice particles and oblate water droplets. Polarization diversity also allows other advances and improvements over single polarization radars.
Polarization diversity can characterize a scatterer by what is known as the full polarimetric covariance matrix, which examines covariances between the co-polar and cross-polar received signals. The matrix consists of the 16 possible covariance combinations of the four possible time series from polarimetric scattering. They are SAA, the signal received on the A polarization channel due to an A polarized transmit signal, SAB, the signal received on the A polarization channel due to a B polarized transmit signal, SBA, the signal received on the B polarization channel due to an A polarized transmit signal, and SBB, the signal received on the B polarization channel due to a B polarized transmit signal. Here polarizations A and B refer to any two orthogonal basis polarizations that can be used. Due to the underlying physics and math, several of the 16 possible values are degenerate. Namely, by reciprocity, SBA and SAB are degenerate, and the covariance commutes (within a sign). This means that only a subset of all possible scattering scenarios and covariance computations are needed to generate the full polarimetric covariance matrix. The full polarimetric covariance matrix allows for complete polarimetric characterization of scatterers.
The covariances reveal the characteristics of the scatterer such as scattering coefficient, Doppler frequency, spectrum width, etc. Scattering coefficient relates to number and size of scatterers, Doppler frequency is directly proportional to the mean radial velocity of the scatterers. Spectrum width relates to turbulence within a sample volume. Some important parameters obtained from the full polarimetric covariance matrix are: Differential phase (ΦDP), Differential Reflectivity (ZDR), Horizontal Reflectivity (ZH), Vertical Reflectivity (ZV), Correlation (ρHV), Linear Depolarization (LDR).
For example, one of the elements of the matrix is the scattering amplitude of the target received on the Horizontal channel when illuminated with a Vertically polarized transmit pulse. Matrix parameters involving both polarizations are known as cross-polar. These polarization products are well known to those skilled in the art and are fully described in the literature, including detailed performance aspects determined through decades of field experiments. Cross-polar measurements are especially useful for particle identification measurements, such as ice detection. Cross-polar measurements require an antenna optimized for cross-polarization isolation (ICPR). Co-polar measurements are generally useful for determining total liquid water content.
Two general methods are used currently to implement polarization diversity. The first method relies on transmitting one of two polarizations in succession (usually alternately). The switching is normally accomplished using a high power A-B switch and an antenna with a separate feed for each polarization. The switch alternatively routes the transmit signal to one or the other of the antenna feeds depending on the polarization desired. Some systems use two high power amplifiers preceded by a similar switching arrangement. In other words the A-B switching is done at low powers prior to being amplified.
Two receivers are used to simultaneously receive co-polar and cross-polar returns for the scatterers. The full polarization matrix can be deduced (within certain limitations and using certain assumptions), but requires twice the number of transmit pulses (since each polarization is alternated) and hence the scan speed must be reduced by a factor of two to regain signal sensitivity and statistics. An additional ambiguity (beyond that given by the Nyquist sampling theorem) exists in the measurement of Doppler velocity using this technique. It is resolved by an assumption of typical scatterer behavior. However, in some cases this assumption is incorrect causing an erroneous Doppler velocity measurement.
A second method (referred to as ‘simultaneous transmit’ or ‘45 degree transmit’) transmits a linear combination of Horizontally and Vertically polarized energy. This is usually accomplished using a high power splitter to simultaneously route the transmitter energy to the two feeds of a dual polarization antenna. A system with two high power amplifiers and appropriate drive circuitry can also be used. The result is in general elliptically polarized, but the signal processing techniques used can easily account for any amplitude and phase offsets encountered. With this technique, many but not all of the parameters of the scattering matrix can be deduced. This technique does not suffer the loss of scanning speed or include the additional ambiguity in Doppler velocity, as does the alternating scheme above. However, certain of the parameters in the scattering matrix that cannot be obtained with this technique are meteorologically significant.
What is needed is a polarization diverse radar system capable of measuring the full scattering matrix of weather targets (unlike the simultaneous transmit technique) without loss of scanning speed and without an additional ambiguity in the Doppler velocity (associated with the alternating polarization technique). It is therefore an object of the invention to provide a polarization diverse radar system to determine the full scattering matrix of scatterers. It is a further object of this invention to provide this measurement without the need to reduce the scanning speed. It is a further object of this invention to provide these measurements without an additional ambiguity in Doppler velocity beyond that required by the Nyquist sampling theorem.